Just four days until Tuesday’s New Hampshire primary, former Massachusetts Gov. Mitt Romney now enjoys a 20-point advantage over his nearest rival in the Granite State, according to a new NBC News-Marist poll conducted after his photo-finish victory in Iowa.

The survey also shows a double-digit bounce since late November in the state for former Pennsylvania Sen. Rick Santorum – who trailed Romney by a mere eight votes in Iowa – as well as a 15-point decline for former House Speaker Newt Gingrich.

And it finds that New Hampshire voters have begun to make up their minds before the Jan. 10 contest, with six in 10 now strongly supporting their candidate of choice.

According to the poll, Romney gets the backing of 42 percent of likely Republican primary voters in New Hampshire, including those leaning toward a particular candidate.

More than a month ago – in an NBC-Marist poll conducted Nov. 28-30 – Romney stood at 39 percent, Gingrich at 24 percent, Paul at 16 percent, Huntsman at 9 percent, Perry at 3 percent and Santorum at 2 percent.

What’s more in this newest survey, 60 percent of likely voters say they strongly support their candidate of choice, up from 49 percent in the last poll. Just 11 percent say they might vote differently.

Electability and ideology help RomneyElectability and the state’s ideological makeup are helping Romney in New Hampshire.

Sixty-five percent of likely primary voters believe that the former Massachusetts governor has the best chance of beating President Barack Obama in November. And more than 60 percent of these voters prefer an electable nominee to one who is a true conservative.

Romney still underperforms among conservative voters, Tea Party supporters and evangelical Christians in the state -- but not to the extent he did in Iowa.

In addition, there are fewer of these more conservative voters in New Hampshire than there were in Iowa.

“New Hampshire is a very different ballgame than Iowa,” says Lee Miringoff, the director of Marist College’s Institute for Public Opinion, which conducted the survey.

Paul’s low ceiling, debates matterWhile Paul sits in second place in the NBC-Marist poll, he has a lower ceiling than Santorum has. Forty-three percent of likely voters find Paul unacceptable, versus 25 percent who say the same about Santorum.

The poll also shows that the debates – and two more of them take place this weekend – have mattered to New Hampshire voters.

A combined 45 percent say they have helped decide their vote a “great deal” or a “good amount.” That’s compared with a combined 10 percent who say the same about the results from the Iowa caucuses, and a combined 20 percent who say the same thing about seeing the candidates in person.

Obama remains under water in New HampshireFinally, President Obama’s job-approval rating in New Hampshire stands at 40 percent among registered voters, which is identical to where it was in late November.

He carried New Hampshire by nearly 10 percentage points in the 2008 general election.

The NBC-Marist poll was conducted Jan. 4-5, and its margin of error for the 2,260 registered voters surveyed is plus-minus 2.1 percentage points. The margin of error for the 711 likely New Hampshire Republican primary voters is plus-minus 3.7 percentage points.